The proper development and function of the human cerebral cortex is controlled by precise regulation of gene expression. Many human neurological disorders, such as epilepsy, schizophrenia and autism, are associated with abnormal structural formation and connection in the cortex. The etiology of these neurological disorders is poorly understood. Recently, a post-transcriptional gene regulation machinery has been identified with the discovery of a class of ~22 nucleotide, endogenous, noncoding, small RNAs, called microRNAs (miRNAs). The mature miRNA controls gene expression by binding to the 3'-untranslated region (3'-UTR) of its target gene and silencing protein translation. Many miRNAs are expressed in the developing brain, but little is known about whether and how miRNAs control development of the cerebral cortex. In this project we will test the hypothesis that noncoding miRNAs maintain the neural progenitor pool and control neuronal production during cortical development. Our hypothesis makes three testable predictions that we will address using neurogenetic and proteomic approaches in the following three Specific Aims. Aim 1 is to test the prediction that microRNA function is required for cortical development. We have created mouse models in which miRNA biogenesis is temporally and spatially blocked in the cortex. We found that the miRNA deficient mouse has smaller cortex. We will determine miRNA function in development of neural progenitors and postmitotic neurons, in cortical neuronal production and anterior and posterior pattern formation. Aim 2 is to examine the distinct role of two specific miRNAs in controlling cortical neuronal production. Using microarrays we identified two miRNAs with temporally differential expression levels during cortical development. We will examine their role in neuronal production by altering their cortical expression levels. Aim 3 is to reveal the mechanisms of miRNA function by identifying their target proteins. We will examine the silencing effect of two specific miRNAs on potential target proteins during cortical development. We will identify new miRNA targets by creating protein expression profiles of miRNA deficient and control cortices using proteomic approaches. The discovery of noncoding microRNAs has revealed a novel machinery of gene regulation. Our project should provide important insights into post-transcriptional gene regulation, which is controlled by miRNAs, in normal brain formation and function. Our results will expand our ability uncover the miRNA-mediated mechanisms that cause mental illness and behavioral abnormalities in humans.